The invention is generally related to integrated optical circuits and components making up such circuits and is more particularly related to active electro-optical silicon devices.
The recent development of low loss single mode optical fibers with low dispersion at the 1.3 and 1.6 micron wavelengths has focused attention on long wave integrated optical circuits and optical systems that couple to such fibers. Such optical circuits and systems are useful in telecommunication, data communication, optical signal processing, optical interconnection, optical sensing, and microwave antenna control applications. Semiconductor guided wave circuits are of special interest because they could, in principle, provide electrooptic integration; that is, the monolithic integration of optical guided wave components with electronic circuits and electro-optical components on a single chip. This application is particularly related to electro-optical components for integration on silicon chips.
The fundamental building blocks of such guided wave circuits are the channel waveguides which are used to make optical components and the interconnects therebetween. It is essential that optical propagation losses be kept to a minimum to allow multiple guided wave components to be cascaded on one wafer (such as in a switching network) without incurring a significant loss penalty. Therefore, an important need is to provide channel waveguides with small physical size so that the waveguides may be densely packed onto the chip. It is also important to provide a fabrication technique that is relatively simple and is capable of fabricating a wide variety of channel waveguide shapes. We have disclosed just such a technique for fabricating a wide variety of channel waveguide shapes in our related application Ser. No. 928,349 filed on Nov. 10, 1986 by Joseph P. Lorenzo and Richard A. Soref. The application is entitled "Method of Fabricating Low Loss Crystalline Silicon Waveguides" and is herein incorporated by reference to this application.
Two prior art waveguide fabrication techniques have been used with some success to fabricate some optical components and waveguides, heteroepitaxy of III-V semiconductors and homoepitaxy of silicon-on-silicon.
Prior art fabrication of III-V semiconductor guided wave components for the 1.3 to 1.6 microns wavelengths suffer from the complexity of using binary, ternary, or quaternary alloy compositions of various materials, and the problems which arise from heteroepitaxy of different volatile materials upon each other. As a result these costly techniques are extremely sophisticated and high quality components are difficult to produce. Further, it is also difficult to add conventional electrical elements to devices so constructed without disrupting the optical structures.
The use of crystalline silicon alleviates most difficulties because the waveguide core uses only an elemental group IV material and all related processing is both highly developed and simple compared to processing of III-V material. Further, use of silicon for electro-optic components facilitates the arrangement of electro-optical components and common silicon electronic components on a single chip.
Epitaxial silicon-on-silicon waveguides and components have been previously constructed. In order to construct such devices a lightly doped silicon layer is grown on a more heavily doped substrate. The refractive index of the substrate is typically 0.01 lower than the index of the epitaxial guiding layer. The problem with such prior art devices is that light tends to leak or "tail" evanescently into the heavily doped substrate because the refractive index difference is insufficient to offer tight mode confinement. Moreover the substrate tends to yield high optical losses due to a large concentration of free carriers therein. As a result, the efficiency of silicon-on-silicon homojunction devices is in the range of 10-15 dB/cm. Losses in this range are not acceptable for medium scale integration of electro-optical components. Also such silicon-on-silicon components are relatively large because it is impractical to shrink the waveguide core area to one micron or less due to losses which result from the extremely high doping of the silicon waveguide "cladding".
In view of the above, a need exists for improved electro-optical components which are not subject to the loss, quality and size limitations associated with prior art devices. A need also exists for improved silicon devices that will simplify the integration of electro-optical and electronic devices on an integrated chip. These silicon devices should be relatively inexpensive to produce and suitable for straightforward and consistent quality manufacture.